As filters and duplexers mounted to communication devices such as mobile phones, used are acoustic wave devices such as surface acoustic wave (SAW) devices and Film Bulk Acoustic Resonators (FBAR). In the acoustic wave devices, a functional element excites an acoustic wave. The functional element in the SAW device is an IDT (Interdigital Transducer) on a piezoelectric substrate while the functional element in the FBAR is a region in which an upper electrode, a piezoelectric thin film, and a lower electrode overlap with each other.
It is important to secure an air-space around the functional element and protect the functional element from moisture and impurities to maintain characteristics of the acoustic wave device good. Japanese Patent Application Publication No. 2006-217226 discloses an invention that seals an IDT with a sealing portion formed of a resin.
However, moisture sometimes penetrates through the resin and enters a space in which the functional element is sealed. Moisture erodes an electrode included in the functional element. In addition, the moisture entering the air-space vaporizes by heating. Increase in volume due to the vaporization increases stress, and causes a crack in the resin. The crack reduces sealing performance and makes the protection of the functional element difficult.